If you need more monitors than your card can support you can add another card in. (Non-SLI.)
Just make sure you get an 8x card or make sure that your 8x slot is open-ended. (See picture.)
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If you want to run SLI: (hopefully getting my second 1080 sometime soon. 144hz gaming ftw.
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Usually if boards are marketed 16/8 or 8/8 they can run SLI, this varies based on board and CPU.
SLI on nvidia requires at minimum an electrical 8x slot.
Crossfire on AMD requires at least an electrical 4x slot.
I say "electrical" because the slots themselves can be up to 16x size, but only have 4x pins in them, in that case, that 16x slot is only 4x electrically.
Certain motherboards / CPU can only operate so many slots at a time depending on the CPU.
If you head on over to
http://ark.intel.com and punch in your CPU. (Intel obviously. AMD have their own site.)
You'll see these specifications:
PCI Express Revision 3.0
PCI Express Configurations ‡ Up to 1x16, 2x8, 1x8+2x4
Max # of PCI Express Lanes 16
The above is for in intel i7 4790K, your CPU may differ.
It mentions the Revision of PCIe, 3.0 being faster than 2.0 and 1.0/1.1, but a PCIe gen 3 card can run in a gen 1 slot, they are backwards compatible.
Lane configuration is the CPU supported PCIe slot configurations that work, in this case I could use 1x16 electrical, 2 x8 electrical, so on.
Max PCIe lanes = Maximum lanes in total to share across slot configurations. (16=16, 8+8=16, 8+4+4=16)
If you post the motherboard model number you can go on the manufacturer website and see what PCIe lane configurations are supported, along with whatever CPU you're using, as long as it means 2x slots are in 8x electrically you're good for SLI.
